Method of investment casting a bright gold alloy

ABSTRACT

An improved gold alloy casting composition wherein sufficient aluminum and/or silicon is utilized in an amount sufficient to form a lustrous oxidation resistant surface coating during and after casting, the method of casting utilizing these components and the cast product. In addition a de-oxidant may also be utilized.

United States Patent Ingersoll Jan. 21, 1975 [54] METHOD OF INVESTMENT CASTING A 2,654,146 10/1953 Mooradian 75/165 X BRIGHT GOLD ALLOY [75] Inventor: Clyde E. lngersoll, Tonawanda, Primary Examiner prancis S. Husar Assistant Examiner]ohn E. Roethcl [73] Assignee: Williams Gold Refining Comp Attorney, Agent, or Firm-Conrad Christel; Edwin T Incorporated, Buffalo, NY. Bean, In

[22] Filed: June 19, 1973 21 A l.N 371517 1 pp 0 57 ABSTRACT Related US. Application Data [62] Division of Ser. No. 221,432, Jan. 27, 1972, Pat. No. An improved gold alloy casting composition wherein sufficient aluminum and/0r silicon is utilized in an amount sufficient to form a lustrous oxidation resis- [52] US. Cl. 164/122, 164/57 t m urfa mating during and after casting, the Int. method of casting utilizing {h g components and the 1 1 Field 01 Search 164/47, 57 cast product. In addition a de-oxidant may also be utilized. [56] References Cited UNITED STATES PATENTS 5/1920 Van Allen 75/165 9 Claims, No Drawings METHOD OF INVESTMENT CASTING A BRIGHT GOLD ALLOY This is a division of application Ser. No. 221,432, filed Jan. 27, 1972, now U.S. Pat. No. 3,769,006.

The present invention is directed to the prevention of the forming of oxide coatings during casting of gold alloys. More particularly the present invention is directed to the use of aluminum and/or silicon in dental and jewelry gold alloys wherein copper, iron, tin, indium or tinlindium mixtures are utilized as the hardening metal. Each further, in the present invention, a minor amount of silicon and/or aluminum is incorporated in the casting composition to provide for a glass-like coating on the surface of the cast product adjacent the mold.

In dental and jewelry gold alloys, the usual hardening element is copper. However, in some cases iron, tin, indium, or tin/indium mixtures are used as the hardening element. When these alloys are cast, e.g., into investment molds in the lost wax process, some of the alloy is partially oxidized during melting and casting procedures. In particular, it appears that at least some of the hardening component is oxidized. The formation of e.g., copper oxide produces a black deposit on the surface of the resulting casting which must be removed by e.g., pickling acid, grinding or sandblasting. After removal of the oxide coating, the dull finish must then be polished in order to obtain a metallic luster on the product.

In order to improve casting processes, and eliminate the above noted problems, many attempts have been made to provide casting compositions or processes whereby the cast, or otherwise molded product has no oxide casting. For example, U.S. Pat. No. 3,613,209 utilizes an oxidation inhibiting metal, inert gas, and injection molding to produce a gold alloy which does not have an oxide coating after molding. However, the above process requires complex and expensive operations such as injection molding under high pressure and the use of inert gas around the melted alloy.

Further, in casting processes, oxygen scavengers such as zinc and indium have been utilized. However, neither zinc nor indium eliminates the dissolved oxides, and neither has a significant effect on the oxide coating of the cast piece.

Another method utilized to at least minimize oxide coating in casting has been vacuum casting. However, the utilization of high vacuums is a relatively expensive operation and such operations have not been wholly successful. Further, various fluxes have been utilized to prevent the formation of oxides during the casting but are normally left in the crucible and thus have not been effective oxidation preventives during solidification and cooling of the alloy in the mold. As a result, graphite has been used in the mold to protect the casting during solidfication and cooling is at least somewhat effective. However, difficulty arises in that some or all of the graphite is burnt out of the investment during the pattern elimination in utilizing the lost wax casting process.

One standard method of minimizing oxide content in cast metal is to add to the melt, an element which has an higher oxidation potential than the element to be protected from oxidation. This is called de-oxidation of the melt and the oxide primarily ends in slag on the surface of the melt. This slag is made up of deliberately oxidized metals, accidentally oxidized metals and slag forming fluxes. However, when such de-oxidation is utilized by the inclusion of, for example zinc and/or indium in noble metal alloys, there is a decrease in the oxide content of the cast alloy but the surface oxide is still present unless the amount of additive utilized is so great as to alter the properties of the alloys drastically. Thus, the surface oxide must still be removed prior to polishing.

Thus, the present invention has as one of its objects the protection of the cast piece from surface oxidation during the important period of solidification and cooling of the alloy in the investment mold.

Another object of the present invention is to minimize still further the inclusion and and/or solution of copper oxide in the melt.

A still further object is to minimize the loss of copper or other hardening elements during the melting and casting of the product.

The present invention has as a further object the provision of a simple process which allows for the substantial, if not complete, elimination of the oxide coating on the surface of the cast product.

Still further the present invention has as its object the production of an alloy which, as cast, has a lustrous gold finish which may be utilized without polishing.

The present objects and others will become clear from the remainder of the present specification.

In the present invention a gold alloy containing for example, up to 50 percent copper or other hardening element is, in the melt form, provided with an amount of silicon and/or aluminum to provide for a surface coating on the finally cast product. Casting is done for example, by the lost wax process and an investment mold and then the product is cooled and removed. The resulting product has a glass-like surface coating which is lustrous in character.

In casting dental or jewelry gold alloys in accordance with the present invention, an alloy containing a hardening element, preferably an element such as up to 50 percent by weight copper, most preferably more than about 5 percent copper, as well as other known additives such as silver, zinc, etc. and has provided additionally, silicon and/or aluminum in an amount sufficient to produce the lustrous cast product of the present invention. The amount of aluminum and/or silicon required varies with the surface area of the mold. For example, when a spherical type mold or a tooth is molded, a smaller surface area is present than for example, when a grid structure is molded. Thus, more of the additive of the present invention is required to produce the lustrous finish of the present invention on a grid structure. However, generally from about 0.01 to about 2 percent by weight based on the total alloy content of aluminum, silicon or both should be provided, preferably from about 0.04 to about 0.5 percent by weight is provided.

Although any conventional hardening elements may be utilized, the preferred group includes copper, iron, tin, indium, and mixtures of tin and indium. When copper is utilized it may be present in amounts up to 50 percent by weight of the alloy, preferably above about 5 percent by weight. When iron is utilized it should be present in amounts up to about 1 percent by weight, preferably from about 0.25 to 0.75 percent by weight. When tin and/or indium are utilized the total amount of hardener, either separately if one is used or combined if both are used, should be up to 7 percent by weight, preferably about 3 to 6 percent by weight.

In addition other conventional elements such as silver, palladium and even platinum may be utilized, Silver is the most common in alloys and is often present in amounts of up to about 30 percent by weight, preferably about 5 to 25 percent by weight when present.

The aluminum or silicon may be added to the melt or to a mixture of the metals as they are being melted in either the pure form, or as master alloys, e.g., alloyed with silver. The alloy of the present invention may also be provided in e.g., bar form, having been previously cast and if needed rolled. In this case the original casting provides the coating, and the subsequent casting also provides a coating. In any case when a master alloy is utilized, the time of addition'of the master alloy is not important. All of the components for the desired product alloy can be placed in e.g., a crucible or other heating means and melted, they may be melted singularly and mixed or melted in one crucible in any serial order. However, when aluminum and/or silicon is added in its unalloyed form, it is preferred that the silicon and/or aluminum be added last.

The alloy melt is then cast by normal procedures, e.g., pouring into a wax investment mold and cooling by standard procedures. The temperature of the melt depends upon the alloy composition and is the same as standard conditions. Generally, however, temperatures from about 1,500" to about 2,500F., preferably about 1,650F to about 2,200F are used. The mold is held at standard temperatures during the period prior to casting, usually about 900F. to about 1,600F., preferably about 1,100F to 1,500F. However, it should be noted that more of the additive of the present invention is required as the mold temperature is increased. In the cooling of the casting, normal procedures are again followed. The mold may be air cooled, quenched or even air cooled to some extent and then quenched.

As to the particular alloy uses contemplated by the present invention, exemplary alloys for dental uses in accord with the present invention are those containing from about 9 to 18 percent copper, as is standard in the art. Further, as to jewelry, the wide variations standard in casting gold alloys for jewelry are possible. That is, normal jewelry alloys will contain from about 92 percent down to about 60 percent gold or lower, although other variations may be possible. 18K gold contains about 75 percent gold and 10 to 20 percent silver in combination with 5 to percent copper, while a 14K dental gold contains normally 63 gold, 30 percent silver and 12 percent copper. The process of the present invention, however, is not dependent upon the various alloy compositions in order to obtain the product of the present invention, but the criticality of the present invention is the provision of sufficient silicon and/or aluminum to produce the glass-like coating on the surfaces of the molded product and thus produce the lustrous product.

Further, as to the other additional components, the

de-oxidants, such as zinc, magnesium, barium, may be abrasion is effected. The glass-like nature of the coating appears to be the best way of describing it, although at the present time its actual character is not known. However, it appears that it may be some form of a silicon or aluminum containing material. If the silicon or aluminum is oxidized as the oxidation potential would indicate, silica or alumina would be produced. However, these compounds melt at above 2,675F. and 3,650F. respectively depending upon their structure and thus it would appear that no liquid form of the silica or alumina would be produced. However, a glasslike coating could be formed at the time of the oxidation reaction. Further, some form of Eutectic may be formed. Therefore, the exact nature of the coating is not clear but it is clear from the experimental results hereinafter disclosed that significant improvements occur by the addition of the present additives in the amounts discussed and that a coating of some nature which is not subject to oxidation is produced.

The present invention will be further shown by the following examples.

EXAMPLE I Table I Element 25C. I350K Calcium l44.4 --l0l.6 Aluminum l25.6 99.0

Copper 35.0 17.8 Silver 4.8 12.4

In these figures it can be seen that both calcium and aluminum, in their oxide forms, are in a much lower en-' ergy state than copper is in its oxide form. Therefore, it would be expected that both calcium and aluminum would oxidize in preference to copper and thus would make good deoxidants.

However, when a dental alloy containing 9.5 percent by weight copper as its only hardening agent (the alloy containing 25 percent silver and 3 percent palladium in addition to the copper and gold) had about 0.04 percent calcium added to it and was cast into an investment mold which was burned out at I,l00F., the re-' sulting product had a gold color but no metallic luster.

On the other hand, when the same 9.5% copper containing alloy having about 0.04 percent aluminum instead of calcium was cast by the same method the resulting alloy had a gold color and was lustrous in the area in contact with the mold.

EXAMPLE II A dental alloy containing 9 percent by weight copper, 12 percent by weight silver and 4 percent by weight palladium was cast after adding 0.02 percent lithium to the melt by pouring it into an investment mold wherein the mold temperature was 1,100F. The resulting cast piece had no black copper oxide on its surface; but had no metallic luster.

EXAMPLE III A dental alloy alloy containing 9.5 percent by weight copper as the alloy of Example I) and 0.64 percent by EXAMPLE IV An alloy of the same composition as in Example 111, except that 0.16 percent tantalum was substituted for the titanium, was cast in the same manner as Example 111 and the results were the same.

EXAMPLE V Another alloy in accord with Example Ill wherein 0.04 percent chromium was substituted for the titanium was cast. The surface in contact with air in this alloy was but 75 percent gray and 25 percent gold color, but without metallic luster. The surface in contact with investment was gold in color but without luster.

EXAMPLE VI Three casting melts with the 9.5 percent copper alloy as in Example I, but with 0.04, 0.08, and 0.228 percent by weight barium were prepared and cast. In all three cases the resulting alloy product had a gold color in the area in contact with the investment, but no luster.

EXAMPLE VII Numerous castings, as set forth in the table below, were prepared with the results as listed; the castings containing 9 percent and 9.5 percent copper are the same as above except for the aluminum or silicon content, while 18 percent copper casting also contains 13 percent silver and 5 percent palladium, the remainder being gold.

From the examples it is clear that aluminum and silicon when used separately produce alloys that are lus trous as cast when very small amounts of either component are utilized. Further, when a more limited range of amounts is utilized the casting not only results in a lustrous surface in the area in contact with the mold, but also results in a lustrous surface in the area which is not in contact with the mold. For example, when 0.04 to 0.15 percent aluminum was used in a 9 percent copper containing gold alloy the resulting casting was lustrous in the area in contact with the mold as well as the area in contact with air. Further, it appears that in addition to being lustrous the castings of the present invention are more easily recovered from the investment mold. That is, there appears to be no adhesion between the mold and the metal and thus the recovery is quite simple. This is possibly due to the lack of wetting of the investment by the molten alloy, although the reasons are not clear at the present time.

EXAMPLE VIII An 18 percent copper containing metal alloy, as in Example VII, was cast with five different aluminumsilicon contents. The contents of the five different examples are set forth below.

Table 11 Copper Element Results Content Added Wt% Casting Air Contact Portion 9% Al 01 Gold-no luster Black 9% Al 02 Gold-lustrous 75% black 9% Al 04 Gold-lustrous Gold-lustrous 9% Al 08 Gold lustr0us G0ld lustrous 9% Al 16 G old-lustrous Gold-lustrous 9% Si 08 Goldlustrous Gold & Black 9% Si 16 Gold-no luster Gold-no luster 9% Si Gold-lustrous Gold-no luster 9% Si 84 Gold-lustrous Gold-lustrous 9.5% Al .04 Gold-lustrous Black 9.5% Al .08 Gold-lustrous Gold-lustrous 10% black 9.5% Al 16 Gold'lustrous Gold-lustrous 10% Gray 9.5% A1 '42 Gold-lustrous Gold-lustrous 9.5% Si 44 Gold-lustrous Gold-no luster.

few black spots gray 9.5% Si 48 Gold-lustrous Gold-lustrous,

20% black spots 9.5% .56 Gold-lustrous Gold-lustrous,

Gray spots 18% Al .06 Gold-lustrous, Black 50% black 18% Al .08 Gold-lustrous Lustrous.

10% black 18% Al .16 Gold-lustrous Gold-lustrous 18% Si .16 Goldmo luster Black 18% Si .32 Gold-lustrous black & black 18% Si .80 Gold-lustrous 50% black & no luster 18% Si .90 Gold-no luster Black Table III Trial Compo- Amounts nents A A1 0.12% Si 0.20% B A1 0.10% Si 0.10% C A1 0.03% Si 0.05% D A1 0.02% Si 0.10% E Al 0.02% Si 0.05%

In all of the above cases the casting had a metallic luster where it was in contact with the invention and varying degrees of blackness where it was cooled in contact with air. From the previous examples it can be seen that where 0.08 percent aluminum is required to produce the lustrous casting, and a completely lustrous casting was not produced utilizing silicon, as little as 0.02 percent plus 0.05 percent silicon produced a completely lustrous casting except for the surface which solidified in air contact. Thus it appears that aluminum and silicon are more than additive in their effects.

EXAMPLE IX In gold base dental alloys containing, in addition to the gold, 3 percent palladium, 8 percent platinum, less than 1 percent indium and tin, and 0.25 percent by weight iron as the hardening agent, normally a dark oxide coating is formed when the lost wax process is utilized for casting. In accord with the present invention, however, varying amounts of aluminum were added to the casting melts with other results. When 0.01, 0.02, and 0.04 percent by weight of aluminum were added to three different casting compositions the resulting product was somewhat lustrous but the investment stuck to the surface of the cast product. However, when 0.08, 0.16 and 0.32 percent aluminum were added the product was lustrous and was easily recovered from the investment.

EXAMPLE X When castings were made in accordance with Example IX, but silicon was used instead of aluminum, 0.02 percent silicon was required to obtain a low luster casting and only 0.08 percent silicon was required to obtain the full metallic luster.

EXAMPLE XI Further dental casting alloys were prepared in accord with Example IX utilizing tin and/or indium as the hardening agent. The casting composition was moditied with aluminum in amounts of from 0.04 to 0.32 percent by weight. The resulting castings were lustrous but the-investment tended to stick.

EXAMPLE XII Alloys of the same composition as those in Example XI were prepared in the same manner except that silicon was substituted for the aluminum in each of the alloys. A casting having 0.04 percent by weight silicon had a low luster, but all of the other castings, i.e., those up to 0.32 percent by weight had high luster and came out of the investment cleanly.

EXAMPLE XIII A 9 V2K jewelry alloy (38.5 percent gold, 50 percent copper, and 1 1.5 percent silver) was modified with aluminum with the following result:

% Aluminum Casting Results 0.02 Black and dull 0.04 Black and dull 0.06 Lustrous with black spots 0.08 Lustrous EXAMPLE XIV Silicon was utilized to modify the casting composition of Example XIII with the following results:

% Silicon Casting Results 0.02 Black and dull 0.04 Black and dull 0.06 Lustrous with black spots 0.08 Lustrous EXAMPLE XV It is apparent from the scanning electron photomicrographs that adding silicon and aluminum to a melt results in a cast product having an oxygen impervious coating or layer on the surface of the casting. This layer is continuous if there is sufficient additive, but discontinuous if insufficient additive (e.g. 0.06% Al or Si in Examples XIII and XIV). The requirements for the amount of additive with regard to the surface area can be seen from the results wherein the dental alloy containing 18 percent copper used above was modified with 0.04 percent aluminum, 0.06 percent silicon, 0.03 percent chromium, and 0.0l percent yttrium. Two types of castings were made with the alloy composition, one being a sphere and the other a grid. The spherical structure obviously has a significantly lower surface area per unit volume than the grid and in these castings was lustrous. However, the grid, although it was quite lustrous showed small black spots which were evident under magnification. Thus the surface coating phenomenon of the present invention is seen and the requirements for sufficient amounts of aluminum and/or silicon to form this surface coating are evident.

What is claimed:

1. A method of investment casting gold alloys at from 1,500 to 2,500 F. comprising providing a melt of gold alloy containing up to about 0.5 percent by weight, based on the total melt, of a material selected from the group consisting of silicon, aluminum and mixtures thereof, said aluminum, if present, being present in an amount of at least 0.02 percent by weight, based on the melt, and said silicon if present being present in an amount of at least about 0.05 percent by weight, based on the melt, pouring said melt into an investment mold, said mold being at a temperature of from 900 to 1,600F. and cooling said mold to solidify said alloy, whereby a cast product having a lustrous surface is provided.

2. The method of claim 1 wherein a hardening element selected from the group consisting of copper,

iron, tin, indium and mixtures of tin and indium is additionally present in said alloy and wherein, when said copper is present, it is present in amounts of from 5 to 50 percent by weight of said alloy, when iron is present, it is present in amounts of up to 1 percent by weight of the alloy, when tin is used, it is present in amounts of up to 7 percent by weight of the alloy, when indium is used, it is present in amounts of up to 7 percent by weight of the alloy and when both tin and indium are used, the total amount of tin and indium is up to about 7 percent by weight of the alloy.

3. The method of claim 2 wherein said material is copper.

4. The method of claim 1 wherein said alloy contains about 0.03 percent by weight chromium.

5. The method of claim 1 wherein said melt temperature is from about l,650 to 2,200F.

6. The method of claim 1 wherein said mold temperature is from 900 to 1,500F.

7. An investment cast gold alloy having a lustrous oxidation resistant surface coating, said gold alloy containing up to about 0.5 percent by weight, based on the total melt, of a material comprising silicon and aluminum, where said aluminum is present in an amount of at least 0.02 percent by weight, based on the melt, and said silicon is present in an amount of at least about 0.05 percent by weight based on the melt, pouring said melt into an investment mold, said mold being at a temperature of from 900F. and cooling said mold to solidify said alloy, whereby a cast product having a lustrous surface is provided.

8. The investment cast alloy of claim 7 wherein a hardening element selected from the group consisting of copper, iron, tin, indium and mixtures of tin and indium is additionally present in said alloy and wherein,

to about 7 percent by weight of the alloy.

9. The investment cast alloy of claim 7 wherein said alloy contains about 0.03 percent by weight chromium. 

2. The method of claim 1 wherein a hardening element selected from the group consisting of copper, iron, tin, indium and mixtures of tin and indium is additionally present in said alloy and wherein, when said copper is present, it is present in amounts of from 5 to 50 percent by weight of said alloy, when iron is present, it is present in amounts of up to 1 percent by weight of the alloy, when tin is used, it is present in amounts of up to 7 percent by weight of the alloy, when indium is used, it is present in amounts of up to 7 percent by weight of the alloy and when both tin and indium are used, the total amount of tin and indium is up to about 7 percent by weight of the alloy.
 3. The method of claim 2 wherein said material is copper.
 4. The method of claim 1 wherein said alloy contains about 0.03 percent by weight chromium.
 5. The method of claim 1 wherein said melt temperature is from about 1,650* to 2,200*F.
 6. The method of claim 1 wherein said mold temperature is from 900* to 1,500*F.
 7. An investment cast gold alloy having a lustrous oxidation resistant surface coating, said gold alloy containing up to about 0.5 percent by weight, based on the total melt, of a material comprising silicon and aluminum, where said aluminum is present in an amount of at least 0.02 percent by weight, based on the melt, and said silicon is present in an amount of at least about 0.05 percent by weight based on the melt, pouring said melt into an investment mold, said mold being at a temperature of from 900*F. and cooling said mold to solidify said alloy, whereby a cast product having a lustrous surface is provided.
 8. The investment cast alloy of claim 7 wherein a hardening element selected from the group consisting of copper, iron, tin, indium and mixtures of tin and indium is additionally present in said alloy and wherein, when said copper is present, it is present in amounts of from 5 to 50 percent by weight of said alloy, when iron is present, it is present in amounts of up to 1 percent by weight of the alloy, when tin is used, it is present in amounts of up to 7 percent by weight of the alloy, when indium is used, it is present in amounts of up to 7 percent by weight of the alloy and when both tin and indium are used, the total amount of tin and indium is up to about 7 percent by weight of the alloy.
 9. The investment cast alloy of claim 7 wherein said alloy contains about 0.03 percent by weight chromium. 